Effective interventions to prevent or delay common chronic conditions and function impairments among the growing older adult population are urgently needed. Physical activity (PA) behavior is important to maintain independence, promote health, and reduce health care costs. Despite recommendations by national organizations, a majority of older adults do not engage in regular PA. The body of literature regarding interventions to increase PA behavior among older adults is rapidly accumulating and considerably diverse. An understanding of the overall effectiveness and patterns of effectiveness of PA interventions in this population is needed to guide future, evidence-supported primary research. The purpose of this study is to conduct a meta- analysis of interventions to increase PA behavior among adults age 65 and older in order to evaluate the overall mean effect size of these interventions. Furthermore, a moderator analysis will be performed to identify specific factors contributing to intervention effectiveness. The moderator analysis will determine which characteristics of interventions are linked with better PA behavior outcomes. Findings from this study will be used to produce future primary PA intervention research and drive clinical practice recommendations to promote PA behavior among older adults. PUBLIC HEALTH RELEVANCE: The growing number of older adults in conjunction with the high rate of chronic illness in this population has a profound impact on health economics and public health. Increasing levels of physical activity in this population may decrease the incidence of chronic illness morbidity and mortality, ultimately impacting health care cost and clinical outcomes. This research project will provide evidence-supported findings to guide both future physical activity intervention research and clinical practice aimed at increasing physical activity levels in this population.